x/crypto/ssh: data race if session.Stdout and Stderr are the same writer

[davecheney]

https://golang.org/cl/9711043/ identified that the ssh package does not handle
the case where the Stdout and Stderr point to the same writer, as in the case of the
CombinedOutput() helper function.

This was worker around by introducing a blocking wrapper, however the proper solution is
to handle this common case the way os/exec handles it.

[davecheney]

Comment 1:

I had a quick look this afternoon and although they share the same API, os/exec an ssh
share a different underlying implementation. When os/exec detects that stdout and stderr
are the same writer, it passes the pipe representing stdout to the underlying exec as
stderr. 
ssh is a little different, we have two handlers for stdout and strerr, this goes all the
way up to the channel level, and they are established quite early. The easiest thing
will probably be to keep kr's singleWriter but make it internal.

[hanwen]

Comment 2:

should this bugreport be closed then?

[rsc]

Comment 3:

Labels changed: added go1.3maybe.

[rsc]

Comment 4:

Labels changed: added release-none, removed go1.3maybe.

[rsc]

Comment 5:

Labels changed: added repo-crypto.